In certain paper products it is advantageous to be able to achieve the highest possible bulk (low bulk density) at a given strength while satisfying high requirements placed on the surface properties of the products. Examples of such products are tissue products, with which high liquid absorption is a preferential property, and paperboard material or so-called liners for corrugated fiberboard boxes, with which a high degree of flexural rigidity is desired.
High bulk is, of course a necessary factor in achieving high liquid absorption. High bulk also contributes positively to the rigidity or stiffness of the board and the liner products. Since high requirements are also placed on the surface properties of this type of product, i.e. properties which will impart smoothness and softness to tissue products and enable print to be applied easily to the surfaces of paperboard and liners the shive content of the pulps used must be extremely low. The requirement of a low shive content and a given lowest mechanical strength has hitherto limited the possibility of using the most extremely long-fiber chemimechanical pulps of low fine-material contents, which provide the bulkiest products. The methods hitherto known for the production of extremely long-fiber chemimechanical pulps have resulted in pulps which are too weak or in which the coarse shive content is much too high.
High yield mechanical and chemimechanical pulps (&gt;88%) are characterized in that the long whole fibers in the pulp (measured for instance as the fraction captured on a 30 mesh (Tyler standard) wire when fractionating in a Bauer McNett-apparatus) have a high flexural rigidity, which is also a prerequisite for manufacturing products which have a very high bulk. In order to produce pulp whose strength properties are sufficiently good for the pulp to be used in the manufacture of tissue, paperboard or liner products for instance, it has also been necessary hitherto for mechanical and chemimechanical pulp to contain a very high proportion of fiber fragments and fine-material, since these materials function as a binder between the long, stiff fibers. When fractionating in a Bauer McNett-apparatus, it has hitherto been considered necessary for the fine-material content, which is normally characterized as the fraction that can pass through a 200 mesh wire (Tyler standard), to be greater than 10%, preferably greater than 12%, in order to be able to obtain strength properties that are sufficiently good for use in tissue, fiberboard or liner products. Another reason why it has hitherto been considered necessary for mechanical or chemimechanical pulps to contain more than 12% fine-material is because at least this amount is formed nevertheless when working the pulp to reduce its shive content (measured according to Somerville with a 0.15 mm mesh width) to levels that are sufficiently low (less than 0.50%, preferably less than 0.25%) to obtain the desired surface properties.
SE-B-397 851 teaches a method of producing a chemimechanical pulp in which the chips are first impregnated with an alkaline sodium sulphite solution and then preheated with steam at 135.degree.-170.degree. C. for about 10 minutes. The following refinement is effected in an open refiner at a temperature slightly above 100.degree.C. The pulp is refined to 400 ml CSF and a very low shive content is obtained. Thus, when practicing this known method it is elected to refine at a relatively low temperature, i.e. a temperature which is much lower than the so-called lignin softening temperature. A relatively high energy input is then required in the refining process in order to obtain a low shive content, which results in a high percentage of fine-material in the pulp. The low shive content is only obtained at a relatively low freeness level. The long preheating time easily leads to a pulp of low brightness, particularly at the longest of these preheating times.
WO-Al-91/12367 describes an absorbent chemimechanical pulp that is manufactured from lignocellulosic material at an extremely low energy input, at a wood yield above 88%, a long fiber content above 70%, preferably above 75%, a fine-material content below 10% and a shive content below 3%. The pulp is produced by preheating and impregnating the chips at high temperature, high pressure and over a short period of time in one and the same vessel, prior to defibering the wood. When producing chemimechanical pulp with the method according to WO-Al-91/12367 at a long fiber content &gt;70% and in which the energy input is maintained at an extremely low level in the refining process, there is often obtained a pulp whose shive content is too high and its strength too low (&lt;10 kNm/kg) for the pulp to be used beneficially in paper products that are required to have high mechanical strength.
By "energy input" is meant in the following the input of electrical energy when refining the fiber material (unless stated differently, the term energy input refers to the total energy input in the single refining stage or in all refining stages). The term "refinement or refining" refers both to the coarse separation of the fibers (defibration) and to working of the fibers (refinement in its true meaning). By "yield" is meant the pulp yield calculated on the fibrous starting material, such as barked wood for instance.